Bad engine vibration -- HELP

Bob,

IMO, the first big question is whether the motor/drivetrain truly vibrates too much, or, are normal vibrations being transmitted too much. I suspect it's the latter: tightening down the motor mounts too much is the surefire way to do this.

I think that if there were mass imbalances in one or more rotating parts, vibration should depend only on rpm; there is no reason for vibration to depend on acceleration/deceleration, or, for the motor/flywheel, whether the clutch is in or out.

Greatest bearing loads and dynamic unbalance vibration commonly occurs during decelerations because the compression strokes are occuring against relatively empty combustion chambers; normally, the fuel/air charge serves as a cushion reducing stress on bearings/rods (interestingly, even more so for high compression heads!). It's quite common for the rear view mirror on a Model A to be useless while backing down from speed.

The only possibility I'm not sure of is the U-joint issue; I would defer to a real mechanic on that but it seems to me it could behave differently between acceleration vs. deceleration loads. But, there's no reason to suspect the U-joint to get worse while your motor was being rebuilt, and every reason to suspect that the motor mounts might be differently snugged.

Loosen up those motor mounts--there may be a really easy fix in your future!

Steve
 
Do you have rear Float-a-motors? Did you check your flywheel housing for square? Both of these can cause the transmission input shaft to not be in direct alignment with the engine crankshaft. Years ago my coupe had FAM's and it looked to me like the rear of the engine was sagging so I jacked it up and put a washer in the FAMs. Then I had a terrible vibration until I loosened the rear spring and let the tension off the drive shaft and retightened it after everything was realigned. A lot of flywheel housings are warped. The service bullitens say if the flywheel housing is out of square more than 6 thousands throw it away - of course that was back in the day when they had lots of them on the shelf.
 
Herm,

I did not see any evidence of removal of metal after the rods were weighed. I showed the guy who balanced the crank, flywheel and pressure plate the orange marks when I brought them in to be balanced. I was not there when they were balanced so I do not know for sure what they did. The pressure plate was on the flywheel when I picked it up after the balancing and the orange marks were lined up.

Larry,

I do not have FAM. I did check the housing alignment when I reassembled the engine.


Bob
 
Front Motor mount

I just removed the NEW reproduction front motor mount kit. Then I re-installed the old parts, springs and flat metal piece. I did not get a new yoke so it is the same one that I have used all along. I did notice that when I removed the front motor mount the rear motor mounts were holding the front of the engine up. After I installed the old front motor mounts I checked the clearance of the yoke to the flat metal piece. I then loosened the rear motor mounts and the gap decreased by about 1/16". So the rear motor mounts were keeping the front of the engine higher than it normally would be. I will take the pickup out for a test run and see what happens with the vibration.

Bob
 
Bob Johnson said:
Herm,

I did not see any evidence of removal of metal after the rods were weighed. I showed the guy who balanced the crank, flywheel and pressure plate the orange marks when I brought them in to be balanced. I was not there when they were balanced so I do not know for sure what they did. The pressure plate was on the flywheel when I picked it up after the balancing and the orange marks were lined up.

Larry,

I do not have FAM. I did check the housing alignment when I reassembled the engine.


Bob
Click to expand...

Bob,
From what I've read, you've done good by your engine redo!
But, from what you've described and have Not done, I would start with what is the easiest, at this point. You've stated that you did NOT check the u-joint condition upon taking the engine out; BUT THAT THE PROBLEM EXISTED BEFORE AND AFTER engine redo.
I'm assuming that you removed the engine by pulling it away from the trans...no? If done this way, you would not have had to touch u-joint ergo checking it out. Before I questioned/suspected a newly redo engine/clutch setup, I'd pull the rear end and check the u-joint for slop AND tightness!
Your stated symptoms(i.e.- vibration principally upon deceleration)IS classic u-joint indicated. Someone mentioned that vibration is NOT dependant upon acceleration nor deceleration...wrong!! Worn/loose u-joint tightens somewhat against itself when power applied , whereas upon deceleration a bad u-joint will flop around like a live fish out of water!
Also when rear pulled, check the drive shaft slop in the bearing inside driveshaft! Good luck
 
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I would be suspicious of motor mounts/ alignment. Do you have rear Floator mounts? . When I reinstalled my engine I got quite a lot of vibration which improved when I adjusted the tension on the floator motor mount bolts
 
Update

As I stated earlier I removed the new reproduction original style front motor mount springs and flat metal plate. I put the old "home made" setup back in. I also loosened the rear motor mount bolts which resulted in the front of the motor dropping about 1/16". Then I re-tightened the rear motor mount bolts.

I drove the pickup about 30 miles on the highway. The vibration problem did calm down but it is still more than before I rebuilt the engine. As before the vibration is the worst when I decelerate. I increased the speed to 65 mph as I approached the off ramp. I took my foot off the gas and left the transmission in 3rd gear (overdrive engaged). At 65 mph the vibration was very slight. As the car slowed down the vibration increased and reached a maximum at about 55 mph at about 45 mph the vibration was very slight again.

As I was going down a fairly steep hill at 55 mph there was considerable shaking (less than before). I pushed in the clutch and as the engine speed decreased the vibration when away. However if I reved the engine up to approximately the same engine speed as it was when the clutch was engaged the vibration returned but was just a little bit less than when the clutch was engaged.

I do have a Mitchell Overdrive so 55 mph is about 1800 rpms.

With the car parked and the transmission in neutral as I reved the engine and the vibration increased up to a certain rpm then calmed down as the rpms got above that speed. I guess that the maximum vibration in this test is the same 1800 rpm that I found in the other tests.

The frequency of the vibration is engine speed dependent and not road speed dependent.

I feel better that the vibration is not as bad as before. But I still think there is something not quite right. It looks like there are 2 issues. First the engine is vibrating too much at a certain rpm. Second that vibration is amplified when the car is decelerating.

What now? Remove engine and recheck all balancing. Remove rear end and check u-joint. Do some more testing ??? Leave it alone and live with the vibration.

Bob
 
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hardtimes said:
Bob,
From what I've read, you've done good by your engine redo!
But, from what you've described and have Not done, I would start with what is the easiest, at this point. You've stated that you did NOT check the u-joint condition upon taking the engine out; BUT THAT THE PROBLEM EXISTED BEFORE AND AFTER engine redo.
I'm assuming that you removed the engine by pulling it away from the trans...no? If done this way, you would not have had to touch u-joint ergo checking it out. Before I questioned/suspected a newly redo engine/clutch setup, I'd pull the rear end and check the u-joint for slop AND tightness!
Your stated symptoms(i.e.- vibration principally upon deceleration)IS classic u-joint indicated. Someone mentioned that vibration is NOT dependant upon acceleration nor deceleration...wrong!! Worn/loose u-joint tightens somewhat against itself when power applied , whereas upon deceleration a bad u-joint will flop around like a live fish out of water!
Also when rear pulled, check the drive shaft slop in the bearing inside driveshaft! Good luck
Click to expand...

H.T., could he jack up one wheel, an turn it back, and fourth, and see how much play is in the joint, might need two people, one at the joint?? Herm.
 
I don't mean to rain on your go-fast "A" model, but from all you have had done to it and you still have a problem [vib's. when i let off the gas at 65 mph] maybe you should go with a V8 and auto. trans. instead of trying to make a 80 year old model "A" 40 hp 4 cly. keep up with todays traffic.
An "A" model is what it is ;).
 
Louis,

The maximum vibration occurs at 55 mph while in overdrive. If I do not use the overdrive the maximum vibration is at about 45 mph. IMHO 45 mph is not an unreasonable speed for a 80 year old Model A.

Bob
 
Bob Johnson said:
Herm,

Since I have a Mitchell installed I have added slop due to the Mitchell.

Bob
Click to expand...

Bob,
I agree with your 'added slop' statement somewhat. When I had the worn u-joint problem and backed off the throttle.....I could actually hear the floping u-joint just barely hitting/ticking the inner cup of the three piece u-joint cover. This was an added piece of evidence..to the vibration. To add injury...there was no lube in the joint, to speak of, when disassembled! What do you have to lose by pulling rear and checking that possibility out first...also as I mentioned the front bearing in the torque tube..just near where the speedo connects.
BTW Bob, you never said...what condition is the rear assemply in?? Torque tube transmits every little bit of bearing/gear wear also!
 
Kohnke Rebabbitting said:
H.T., could he jack up one wheel, an turn it back, and fourth, and see how much play is in the joint, might need two people, one at the joint?? Herm.
Click to expand...
Herm,
Bobs' problem is serious enough to give an upset stomach, i.e.- he has obviously spent bundles of $, time and effort on this second go round with this engine! By his words, sounds like he is questioning the work results on the engine(what could/would,etc)....when it may not be the engine at all. Sounds like it was done right!
You might be right, in turning back/forth, as the u-joint should 'buck' one way. But, now he says overdrive in between which , I see as an additional impediment to back/forth method. Heck, is it easier to pull the engine out again FIRST and tear it down? I'm just saying , rear end is a lot easier to take down and check and go forward....no?
 
Vibration frequency

I would think that if the u-joint was causing the vibration the frequency of the vibration would be dependent on the mph of the car. In my case the frequency of the vibration is linked to the engine speed and not the mph. Is this a correct assumption? Or does the engine vibration somehow amplify through a bad u-joint?

Bob
 
Ray,

Yes I feel a similar vibration in 2nd gear at around 25 mph. Also in 1st and reverse at much lower speeds.

Bob
 
Slop video

Here is a video showing the slop in the driveline. The transmission is in 3rd, the overdrive is in high, the left rear wheel is off the ground and the right rear wheel is on the ground. Maybe someone can see something that will help figure out the condition of the u-joint. It is a Windows Media Video and is about 1 MB. So it may take a while to download depending on your download speed.

Click here for video


Bob
 
Not sure but what if you pulled the "clam shells" off is there enough room to get fingerds in to feel around or have someone turn rear wheel (car jacked up and front wheels blocked) to see if there is excess play in the joint??
Paul in CT
 
1931 flamingo said:
Not sure but what if you pulled the "clam shells" off is there enough room to get fingerds in to feel around or have someone turn rear wheel (car jacked up and front wheels blocked) to see if there is excess play in the joint??
Paul in CT
Click to expand...

The rear spring is too strong to allow that much rearward movement.
 
Re: Slop video

Bob Johnson said:
Here is a video showing the slop in the driveline. The transmission is in 3rd, the overdrive is in high, the left rear wheel is off the ground and the right rear wheel is on the ground. Maybe someone can see something that will help figure out the condition of the u-joint. It is a Windows Media Video and is about 1 MB. So it may take a while to download depending on your download speed.

Click here for video


Bob
Click to expand...

Bob, it looks like to me, with slop in your over drive, and rear end, it is about right. To me it looks like 2" total, and the rear end should be one???

I am going to read all your post again, I'll be back.


Herm.
 
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